Smearing themselves with blood and feces: scientists explain how some snakes fake their death in front of predators

Snakes escape from predators by pretending to be dead, and make their performances even more convincing by smearing themselves with their own blood and feces. This behavior is known as death simulation or thanatosis.

Such play is common in the animal kingdom. In an article published in the journal Biology Letters, researchers described how dice snakes (Natrix tessellata) take their "Shakespearean theater" to a new level.

When snakes encounter a predator, they spew feces and musk, a putrid byproduct of digestion. Then they fall on their backs and pretend to be dead, and sometimes they start bleeding from the mouth. This is probably an attempt to make themselves less attractive to prey.

Risky behavior

"Imitating death is a high-risk, high-reward behavior. If you fake death, you are completely helpless. Therefore, it would be good if you had some kind of protection against being instantly eaten," Vukašin Bjelica, lead author and a biologist at the University of Belgrade, said.

Dice snakes reach a maximum length of about 1.2 meters. They are nonvenomous, so they have few defenses against large mammals and birds in their native range in Europe, Asia, and northern Africa.

But by using a set of behaviors known as a "defense portfolio," dice snakes can make themselves appear unappetizing. The stench of feces and musk, as well as the bloody saliva flowing from the mouth, gives the impression that the snake is a rotten carcass rather than a fresh piece of meat.

The disgust and confusion that this gruesome sight causes in a potential predator can give the snake enough time to escape or even encourage it to abandon its efforts entirely.

Scientists have been monitoring populations on Lake Prespa on the island of Golem Grad in North Macedonia. Snakes were more prone to thanatosis here. The reason for this is a peculiar set of circumstances in which the local reptiles live.

"Firstly, it is the intense pressure of bird predation. Secondly, these snakes are quite large compared to their mainland relatives. Being big is only good when you're facing a predator with a small open mouth," Bjelica explains.

Scientists tested 263 snakes on the island. They handled the animals to simulate how a predator might approach its prey: grabbing, holding and stretching their bodies. Of these snakes, 124 smeared their bodies with feces. Twenty-eight bled from the mouth. This is a phenomenon called autohaemorrhaging or reflex bleeding.

Bjelica believes that individual characteristics of snakes can determine their behavior.

"Some individuals are more daring and risk-taking, while others are shy, so they are less likely to behave this way," he said.

Only one young dice snake was bleeding from the mouth, indicating that thanatosis is more common in adults. For smaller ones, it may be too much of a risk.

Snakes are not the only animals that pretend to be dead. Virginia opossum (Didelphis virginiana) can defecate and salivate to simulate death, which suggests that this evolutionary strategy has proven useful across the entire spectrum of the animal kingdom.

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